1. Field of Invention
The present invention relates generally to the Synchronous Optical Network (SONET), and more particularly to Bi-directional Line Switched Rings (BLSRs) for SONET.
2. Related Art
A conventional asynchronous digital hierarchy for carrying digitized voice signals over twisted wire consists of digital stream (DS) levels, with a DS0 signal carrying a single voice channel and being the lowest level in the hierarchy with a bit rate of 64 kbps. The lower level digital signals are multiplexed into higher level digital signals. For example, 24 DS0 signals are multiplexed to form a DS1 signal, which has a bit rate of 1.544 Mbps and carries 24 single voice channels, and 28 DS1 signals are multiplexed to form a DS3 signal, which each have a bit rate of 44.736 Mbps and carry 672 (24*28) single voice channels. Currently, multiplexing to form DS3 and lower level signals is asynchronous and uses multiple stages. For example, when DS1 signals are multiplexed into a DS2 signal, extra bits (bit stuffing) are added to account for data rate variations in the individual signals. Then, when the DS2 signal is multiplexed into a DS3 signal, bit stuffing is again used.
While bit stuffing allows independently clocked input signals to be transmitted, bit stuffing also makes it nearly impossible to locate individual DS1 or DS0 signals/channels within a DS3 signal. To extract a single channel, a DS3 signal would need to first be demultiplexed into 28 DS1 signals before the channels could be switched or rearranged. The appropriate DS1 signal would then also need to be demultiplexed to locate the desired DS0 signal. As a result, the process of adding or deleting channels is expensive and inefficient. Asynchronous multiplexing also increases overhead and requires a large number of multiplexers and digital cross-connects.
Recently, a standard called Synchronous Optical Network or SONET has been developed for carrying synchronous signals with different bit rates and capacities between different fiber optic systems using a byte-interleaved multiplexing scheme. SONET was developed, in part, because of the numerous advantages of data transmission through fiber optic systems, such as very high bandwidth capacity and long communication distances without repeaters or regenerators.
SONET defines various levels of Synchronous Transport Signals (STS), with the lowest level or base signal designated STS-1 having a data rate of 51.84 Mbps. Higher level signals, STS-N, are integer multiples of the STS-1 signal and are formed by byte-interleaving N STS-1 signals. SONET network elements (NEs) combine STS-1 signals as needed to form an STS-N signal and then convert the STS-N electrical signals to an Optical Carrier (OC) signal and transmit the OC-N signal over optical fiber.
In existing networks when there is a need to extract a lower speed signal from a high speed line or insert a lower speed signal into a high speed line, the lower speed signals must be completely demultiplexed or multiplexed, respectively, before being passed on. Multiplexing and demultiplexing can become very expensive as the number of low speed signals that are used increases. SONET overcomes this problem through the use of synchronous multiplexing and Add/Drop Multiplexers (ADMs), which allow low speed signals to be added or dropped without demultiplexing the entire signal.
In contrast to asynchronous multiplexing, the SONET standard supports greater capacity and efficiency. In the SONET multiplexing format, the basic signal transmission rate STS-1 operates at 51.84 Mbps. An STS-1 can carry 28 DS1 signals or one asynchronous DS3 signal. STS-1 signals are then multiplexed to produce higher bit rate signals STS-2, STS-3, etc. As mentioned above, the other term used to define SONET signal levels is Optical Carrier. The bit rates are the same with both STS and OC signals, i.e., the bit rate of the STS-1 equals the bit rate of the OC-1, with the only difference being the type of signal that is referenced. For example, if the signal is in an electrical format, the signal is referred to as STS, while, if the signal is in an optical format, the signal is referred to as OC.
Regardless of whether STS or OC signals are being transported by SONET, a major objective with SONET is to increase the survivability of the network. Because of the large bandwidth capabilities with optical fiber and the growing volume of data traffic, data transmission disruptions can result in serious consequences with the operation of the network. Communication between two nodes or links can be disrupted due to cable cuts or node failures, for example. Several types of network topologies have been used to enhance the survivability of the network, such as point-to-point systems incorporating route diversity, in which two nodes employ an alternate communication path with regenerators in the event the primary path experiences a communication disruption. These types of topologies, while designed to be survivable, can increase the number of multiplexers and the length and number of cable required, as well as possible network elements, such as regenerators or optical amplifiers in order to make the necessary connections between communication nodes.
A much more preferred topology for SONET are self-healing ring (SHR) architectures, in which the nodes are connected in a ring configuration. Duplicate signals are transmitted along two paths, a working path and a protection path. When a communication is disrupted along the working path, the protection path is selected to allow the data transmission to continue to the desired destination node or nodes. SHRs include uni-directional path switched rings (UPSRs) and bi-directional line switched rings (BLSRs).
For USPRs, adjacent nodes on the ring are connected with a single optical fiber carrying data through the nodes of the network in a clockwise direction and a single optical fiber carrying data in a counter-clockwise direction. One fiber represents the working path and the other represents the protection path. For BLSRs, the working path and the protection path travel along the both directions in the ring. If a fiber cut in the working path or a node failure occurs, data transmission is switched to the protection path. Such architectures take advantage of the capability provided by synchronous multiplexing in SONET to eliminate the need to backhaul traffic to central hubs. Thus, at each switching node, the SONET transport node directly accesses the required time slots in the bit stream through the use of modified (ADMs).
SONET ring topology permits the creation of highly survivable networks and results in cost savings since it can be much less expensive for carriers to install a fiber ring than to deploy point-to-point links. Due to the advantages of BLSR configurations for SONET networks and an increasing number of users subscribing to SONET networks, more and more traffic terminating nodes may he desired or required on a single BLSR network. However, current SONET ring topology for BLSRs only allow a ring configuration having a maximum of sixteen traffic terminating nodes.
Accordingly, a SONET BLSR topology is desired which is allows more than sixteen nodes to be connected in a ring network.
The present invention provides a method and structure to allow more than 16 nodes on a SONET bi-directional line switched ring (BLSR) network by utilizing unused portions of the transport overhead to expand the node identification field from four to eight bits.
In one embodiment, a third APS channel byte K3 is allocated to the byte location at the fifth row and second column of the transport overhead from the second STS-1 signal. In the current SONET BLSR standard, the destination and source nodes on the ring are identified by the four least significant bits (0-3) of the K1 byte and the four most significant bits (4-7) of the K2 byte, respectively, which allows a maximum of 24 or 16 nodes to be identified on the ring. The node identification field is expanded by using the four least significant bits (0-3) of the K3 byte for the K1 node identification and the four most significant bits (4-7) of the K3 byte for the K2 node identification. As a result, node identification is expanded to 28 or 256 possible values, thereby allowing up to 256 nodes on a single ring.
The present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.